Anemia generally refers to any abnormality in hemoglobin or erythrocytes that leads to reduced oxygen levels in the blood. Anemia can also develop in association with chronic diseases, e.g., chronic infection, neoplastic disorders, chronic inflammatory disorders, including disorders with consequent inflammatory suppression of marrow, etc. Anemia of chronic disease is one of the most common syndromes in medicine.
Anemia of chronic disease (ACD) is often associated with iron deficiencies. ACD can develop from inadequate availability of iron (e.g., anemia of iron deficiency) or, in cases where total body iron is adequate but the requirements for hemoglobin production are defective (e.g., functional iron deficiency). Iron is required for production of red blood cell hemoglobin in erythropoietic precursor cells of the bone marrow.
Numerous physiologic deficiencies are observed in patients with anemia of chronic disease, including reduced erythropoietin (EPO) production, reduced EPO responsiveness of the bone marrow, and reduced iron metabolism, including reduced iron absorption from the gut, reduced iron trans-enterocyte transport, reduced iron oxidation to the ferric state by hephaestin or ceruloplasmin, reduced iron binding and uptake, by transferrin and transferrin receptor, and reduced iron transport to the marrow where iron utilization occurs, including heme synthesis. Individually and together, these physiologic deficiencies contribute to ineffective or impaired erythropoiesis, which can lead to microcytic anemia and hypochromic red blood cells associated with reduced hemoglobin production and reduced oxygen transport.
Anemia of chronic disease is associated with increased production of inflammatory cytokines (Means (1995) Stem cells 13:32-37 and Means (1999) Int J Hematol 70:7-12), including, for example, tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), IL-6, and interferon-γ (IFN-γ). In several in vitro and in vivo animal model systems, inflammatory cytokines negatively affected the ability to mediate EPO production, EPO responsiveness, and the coordinate regulation of iron metabolism (Roodman et al. (1989) Adv Exp Med Biol 271:185-196; Fuchs et al. (1991) Eur J Hematol 46:65-70; Jelkmann et al. (1994) Ann NY Acad Sci 718:300-311; Vannucchi et al. (1994) Br J Hematol 87:18-23; and Oldenburg et al. (2001) Aliment Pharmacol Ther 15:429-438.) Administration of erythropoietin failed to reverse anemia in mice continuously exposed to TNF-α (Clibon et al. (1990) Exp Hematol 18:438-441). Increased levels of inflammatory cytokines, such as TNF-α, IL-1β, and INF-γ, contribute to defective EPO production and EPO resistance observed in patients with anemia of chronic disease (Jelkmann et al. (1991) Ann NY Acad Sci 718:300-311 and Macdougall and Cooper (2002) Neprol Dial Transplant 17(11):39-43). Therefore, various cytokines, e.g., inflammatory cytokines and cytokines associated with inflammation, are involved in many aspects of the pathogenesis of anemia of chronic disease, including inhibition of erythroid progenitors, inhibition of EPO production, and impairment of iron release and iron availability for erythropoiesis.
There is thus a need in the art for methods of treating or preventing anemia of chronic disease. There is a need in the art for methods of overcoming the deficiencies in current use of recombinant EPO to treat anemia of chronic disease. In particular, there remains a need for methods and compounds effective at overcoming suppressed EPO production and decreased EPO responsiveness associated with anemia of chronic disease, for methods and compounds effective at enhancing regulation of iron metabolism and overcoming deficiencies of altered or abnormal iron metabolism and utilization, and for methods and compounds effective at enhancing total or complete erythropoiesis by improving the metabolic pathways related to EPO production, EPO responsiveness and signaling, and iron availability, utilization, uptake, transport, processing, etc. There is a need in the art for methods of overcoming or of ameliorating the consequences of cytokine-induced effects in subjects having anemia of chronic disease.
Iron deficiency is one of the most common nutritional deficiencies worldwide and is the leading cause of anemia on a global basis. Iron balance is fundamentally regulated by the rate of erythropoiesis and the size of iron stores. Iron deficiency can occur with or without anemia, and has been associated with impaired cognitive development.
Iron deficiency is defined as inadequate iron supply (levels or stores) or as inadequate availability or utilization of iron in the body. This can be due to nutritional deficiencies, e.g., lack of iron in the diet; to iron malabsorption, due, for example, to surgery (post-gastrectomy) or disease (Crohn's disease); or to a depletion in iron supply or increased iron loss due to chronic or acute blood loss resulting from injury or trauma, menses, blood donation, phlebotomy (such as due to various procedures, surgeries); from increased iron demand, e.g., due to rapid growth in infancy or adolescence, pregnancy, erythropoietin therapy, etc.
Iron deficiency can also be functional iron deficiency, e.g., iron deficiency characterized by the subject's impaired ability to access and utilize iron stores. Iron is not available at a rate sufficient to allow normal hemoglobinization of erythrocytes, leading to reduced reticulocyte and erythrocyte cellular hemoglobin content. Functional iron deficiency is often seen in healthy individuals with apparently normal or even increased iron stores but with impaired iron availability, as measured, e.g., by low levels of percent transferrin saturation. This type of iron deficiency is frequently associated with acute or with chronic inflammation.
Iron deficiency of any kind can lead to iron-deficient or iron-restricted erythropoiesis, in which red blood cell numbers decrease and circulating red blood cells are smaller than normal (microcytic) and lack adequate hemoglobin, and as such are pale in color (hypochromic).
Subjects with iron deficiency, including functional iron deficiency, can develop impaired hemoglobin synthesis, reduced % transferrin saturation, and decreased hemoglobin and hematocrit levels, leading to iron deficiency anemia. Iron deficiency anemia is the most common anemia in the world. Iron is an essential component of hemoglobin; without iron, the marrow is unable to produce hemoglobin effectively. Iron deficiency anemia may occur in subjects with depleted or impaired iron supply, or may occur in subjects having functional iron deficiency, when iron is present in storage but is unavailable, e.g., for hemoglobin production.
In view of the above, there is a need in the art for methods of treating or preventing disorders associated with iron metabolism, and a need in the art for methods of enhancing iron metabolism. There is a need for methods of treating or preventing iron deficiency, including functional iron deficiency, and for treating or preventing associated conditions such as microcytosis and iron deficiency anemia.
The present invention provides methods and compounds for enhancing the metabolic and physiologic pathways that contribute to complete and effective erythropoiesis, and in particular, for treating anemia of chronic disease. Methods and compounds for overcoming the suppressive/inhibitory effects of inflammatory cytokines on EPO production and responsiveness are also provided. Additionally the present invention provides methods and compounds for enhancing iron metabolism, and for treating or preventing conditions associated with impaired iron metabolism, such as iron deficiency, including functional iron deficiency, iron deficiency anemia, microcytosis, iron-deficient erythropoiesis, etc.